Cyclic uterine differentiation will be analyzed in the albino rat and human being, primarily through electron microscopy used in association with radioautography and cytochemistry. In both species, ultrastructural analysis will center on the luminal epithelium, endometrial stroma, and myometrium. The sequential differentiation of the lysosomes will be followed in luminal and glandular epithelia through ultrastructural localization of acid phosphatase. The precursors of epithelial cells (cyclic rat) will be identified through H3-thymidine incorporation, and their differentation and fate followed. In the immature rat uterus, biochemical responses of the precursor populations to various estrogens will be studied in relation to simultaneous ultrastructural differentiations of cells and tissues. Stromal analysis will, in particular, identify cyclic variation in the cells associated with immune responses, lymphocytes, monocyte-macrophages, and plasma cells. Experimental approaches will be used to identify the intertissue migrations of the uterine monocyte-macrophage. In the human endometrium, effort will center on the lamina basalis as the germinal bed for the functionalis and on the cyclic changes in the ultrastructure of the spiral arteries in comparison with the basal arteries. Endothelial phosphatases will be localized to assist in the identification of the smallest branches and special capillary terminations of the spiral artery.